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Fig. 28.—The Cocoon of the Emperor Moth.

Fig. 29.—The Cocoon of the Six-spotted Burnet (Filipendulæ).

We have seen that the Large White Butterfly makes itself secure by a silk band round its middle, while the 'Tortoiseshell' is fixed only by its tail. But the extra provision for the safety of the former is not so necessary in the case of the latter, as it never spends more than two or three weeks in the pupal state. Here it is the perfect insect that braves the winter, and not the chrysalis.

There is a great variety in the means taken by the caterpillars of moths to protect themselves during their metamorphoses, but we shall have space for only a few illustrations.

A clever cocoon is spun by the larva of the Emperor Moth (Pavonia). It is pear-shaped, and composed of a brownish silk; and is so constructed that the newly emerged moth can easily walk out of the small end without breaking a fibre, while the entry of an insect enemy from without is impossible.

This is managed as follows. A number of rather stiff threads are made to project from the small end of the cocoon, and these converge as they pass outward so that the ends are all near together. The other portions of the cocoon are of compact silk, and any insect intruder that ventures to enter by what we may almost term the open end is met by a number of spikes, as it were, that play on it at every attempt. Many of these wonderful cocoons may be found during the winter months attached to the food plants of this insect.

Of the silken cocoons spun by various caterpillars some are so thin and light that the chrysalis can easily be seen through them, and others are so densely woven as to be quite opaque. A great difference is also to be observed in the adhesive power of the silk fibres. In some cases little threads of silk can be pulled off the cocoon; but some of them, that of the Oak Eggar (page 229) for example, look as if they had been constructed of paper rather than of silk, because, at the time of spinning, the moist silk fibres stuck so closely together.

An extreme case of this character is to be met with in the cocoon of the Puss Moth (page 235); for here the fluid from the spinneret of the caterpillar does not harden at once on exposure to air, and so the threads become thoroughly united together, thus forming a solid gluey cocoon.

When the Puss caterpillar is about to change, it descends the tree (poplar, willow, or sallow) till it is within a few feet of the ground. Then it commences gnawing away at the bark, at the same time cementing all the pieces together with the gluey substance from its spinning glands. In this way it surrounds itself with a very hard cocoon, which so closely resembles the surrounding bark in colour that detection is difficult indeed.

But how will the caterpillar proceed if it is removed from its native tree and has no bark to gnaw? That you can easily answer for yourself, or rather Puss will answer it for you. Go and search among the poplars, willows and sallows in the month of July. You may possibly come across a caterpillar that is just in the act of creeping down the bark in search of a resting place; but if not you may be successful in obtaining a few either by examining the twigs, or you may start them from their hiding places by smartly tapping the smaller branches with a strong stick.

Having secured one or more larvæ, take them home, and they will give some rather novel performances. If they are not fully grown, you must supply them with fresh leaves every day till they refuse to eat; and then is the time for your experiments. Shut one in a little wooden box, and you will have the pleasure of watching it construct a cocoon of chips of wood that it has bitten out with its powerful jaws, all joined together into a hard shell by means of transparent glue. Shut another Puss in a glass vessel—a tumbler, for instance—either by placing it under the inverted vessel, or by covering over the top. Perhaps it will not be superfluous to mention that, should you place it under an inverted vessel, this vessel should not stand on a polished table, for, whatever be the material, unless extremely hard, it is sure to be utilised in the manufacture of the cocoon.

Let us suppose, then, that the caterpillar is under an inverted tumbler that stands on a plate or saucer. Now it is for you to decide what material shall be used in the construction of the new home. Give Puss some fine strips of brightly coloured ribbon, and it will construct a very gaudy house by gluing them together. Or, provide it with sawdust, pieces of rag, glass beads, sand, paper, anything in fact; and the material will be 'made up' into a cocoon more or less ornamental according to the nature of the supply.

But what if you give it nothing with which to work, and so inclose it that nothing its jaws can pierce is within its reach? For instance, shut it in with tumbler and saucer as before, inverting the former on the latter, and give it no material whatever. What will it do now? We will watch and see.

At first it is very restless, and walks round and round the edge of the tumbler, evidently a little dissatisfied with the prospects. Then, after a little while, the events of nature transpiring in their fixed order regardless of trivial mishaps, the glutinous fluid begins to flow from the creature's spinning glands, and it moves about in a somewhat aimless fashion, applying the transparent adhesive matter both to tumbler and saucer.

It seems now to become a little more reconciled to its unnatural surroundings; and, making the best of bad matters, keeps its body in one place, and starts the construction of a ridge or barrier all round itself. By the continued application of the creature's spinneret this barrier is made gradually thicker and higher, till at last the overhanging sides meet and the caterpillar is inclosed in its self-constructed prison. But the walls of this prison are so transparent that every movement can be watched; and, after the insect has spent a few days in completing the cocoon, we can see it cast off its old skin, and appear in the new garb of a fine greenish chrysalis.

Its soft green skin soon hardens and turns to a rich dark brown colour, and it settles down for a long rest lasting till the following May or June.

When the whole operation of building is completed, lift up the tumbler, and up will come the saucer too. The two are firmly glued together by the substance secreted; and the power of this as a cementing material will be well illustrated if you endeavour by mere pulling force to separate the two articles.

The Puss is not the only caterpillar that works up a foreign material with the contents of the spinning organs. There are several others, in fact, that use for this purpose fragments of wood or other parts of the food plants; and a still larger number bind together leaves, fresh or dead, or particles of earth or other matter. Several such cocoons will be described in our accounts of individual species in another chapter. We shall now devote a little space to a few general remarks on the chrysalides and the final metamorphosis of butterflies and moths.